Frequency modulated radio frequency broadcast network employing a synchronous frequency modulated booster system

ABSTRACT

A frequency modulated ratio frequency broadcast network in which re-transmitting stations employ a synchronous frequency modulated booster system. Included in the frequency modulated booster system is a synchronous frequency modulated exciter for converting a frequency modulated intermediate frequency into a frequency modulated broadcast signal. The synchronous frequency modulated exciter includes phase locked loop circuits for synchronizing the re-transmitted frequency modulated broadcast signal with the frequency modulated intermediate frequency signal by detecting reference signals in the composite baseband of the intermediate frequency signal and in the composite signal frequency modulated broadcast signal.

RELATED CASE

This application is a continuation-in-part of our pending application,Ser. No. 07/326,744, filed on Mar. 21, 1989, for Frequency ModulatedRadio Frequency Broadcast Network Employing A Synchronous FrequencyModulated Booster System.Iadd., which has issued into U.S. Pat. No.5,046,124 on Sep. 3, 1991.Iaddend..

BACKGROUND OF THE INVENTION

The present invention relates in general to a frequency modulated radiofrequency synchronous repeater system for the transmission of frequencymodulated broadcast signals, and more particularly to a frequencymodulated radio frequency repeater system employing a synchronousfrequency modulated booster system for the re-transmission of frequencymodulated broadcast signals.

Frequency modulated broadcast transmissions have been limited toaudiences within a reception area. In order to increase the receptionarea for the frequency modulated broadcast transmission to reach agreater audience, re-transmission sites have been installed in areasremotely located from the originating transmitter. At there-transmission sites were booster transmitters and synchronoustransmitter exciters to increase the power level of the frequencymodulated broadcast signals at a re-transmission site.

Heretofore, a demodulation process was used at a re-transmission sitewhich caused frequency discrepancies, phase shifts, delays andinconsistent modulation levels between the originating frequencymodulated broadcast signal and the frequency modulated broadcast signalat the re-transmission site. Phase shifts, frequency discrepancies,delays and inconsistent modulation levels between the originatingfrequency modulated radio frequency signal and the re-transmittedfrequency modulated ratio frequency signal resulted in signaldegradation and noise interference in the coverage area intended forimprovement by the re-transmitted frequency modulated broadcast signalof the booster transmitter.

Heretofore, a device was employed to produce a reference signal forsynchronizing and stabilizing the output of frequency modulated radiofrequency signals transmitted at a re-transmission booster site. Thedevice did not use a reference signal generated at the site of theoriginating transmitter or re-use the FM modulation in the originalcarrier. Hence, the output of the frequency modulated ratio frequencysignal of the transmitter at the re-transmission booster site wasmodified from the original FM signal in frequency and in phase.

In the U.S. Pat. No. to Wu et al., 4,710,970, issued on Dec. 1, 1987,for Method Of And Apparatus For Generating A Frequency ModulatedUltrahigh Frequency Radio Transmission Signal, there is disclosed anultra high radio frequency transmitter. The output of a very highfrequency voltage controlled generator is phase locked through a phasedetector with a voltage controlled crystal oscillator producing areference signal for the stabilizing of the output transmissionfrequency of the ultra high radio frequency transmission frequencyoscillator.

In the U.S. Pat. No. to Martinez, 4,208,630, issued on June 17, 1980,for Narrow Band Paging Or Control Radio System, there is disclosed aradio system for paging in which a central transmitting device andremote receiving devices are phase locked to a local broadcast stationradio frequency carrier so as to provide a means to synchronize thetransmitting device with the receiving device.

The British Patent to McGraw-Edison Company, No. 2,061,581B, publishedon May 18, 1983, for Communication System For Distribution AutomationAnd Remote Metering, discloses a phase detector to which is applied theoutput of a limiter-amplifier and a reference signal from a voltagecontrolled crystal oscillator. The output of the phase detector is acontrol signal which is proportional to the phase differences of theinput signals to the phase detector. The error signal is applied to thevoltage controlled crystal oscillator. The circuit described is part ofa phase locked loop circuit.

In an article published by Omega International of Irvine, Calif.,entitled Synchronous Repeaters, there is mentioned that the outputfrequency of a booster is phase locked with the originating stationthrough analog simulation of a digital control signal derived from theoriginating station.

Heretofore, FM exciters were sold to accept the composite basebandsignal from a stero generator, and STL system or monaural audio and SCAprogramming, and to generate its operating frequency with a digitallyprogrammed, phase-locked frequency synthesis system. Such an FM exciterwas sold by Continental Electronics Mfg. Co. of Dallas, Tex., as theContinental Type 802A FM exciter, and by Broadcast Electronics ofQuincy, Ill., as the Model FX-30.

SUMMARY OF THE INVENTION

A synchronous frequency modulated booster system for a transmitter thatre-transmits frequency modulated radio frequency signals at a boostersite away from the originating program source. The synchronous frequencymodulated booster system includes a synchronous FM transmitter exciterthat converts incoming frequency modulated intermediate frequencysignals transmitted at the site of an originating transmitter to thefrequency modulated ratio frequency signals transmitted by theoriginating transmitter and applies the frequency converted frequencymodulated radio frequency modulated radio frequency signals to a boostertransmitter that re-transmits the frequency modulated radio frequencysignals.

A synchronous frequency modulated booster system comprises a transmitterthat re-transmits frequency modulated radio frequency signals. Thesynchronous frequency modulated booster system includes a synchronous FMexciter that receives an IF signal and a reference pilot signalgenerated at the originating transmitter for synchronizing the carrierfrequency and modulation level of frequency modulated ratio frequencysignals applied to the transmitter for re-transmission with thefrequency modulated radio frequency signals transmitted at theoriginating transmitter.

An object of the present invention is to provide a method ofsynchronizing frequency modulated booster system for a transmitter thatre-transmits frequency modulated radio frequency signals, which boostersystem includes a synchronous FM exciter for stabilizing the frequencyof the booster transmitter and the baseband, and to obviate groupdelays, and inconsistent modulation levels between frequency modulatedradio frequency signals transmitted by an originating transmitter andthe frequency modulated radio frequency signals re-transmitted from aremote transmitter.

Another object of the present invention is to provide a synchronousfrequency modulated booster system for a transmitter that re-transmitsfrequency modulated radio frequency signals which booster systemincludes a synchronous FM exciter for receiving a reference signal fromthe site of the originating transmitter along with frequency modulatedintermediate frequency signals for synchronizing the re-transmittedfrequency modulated radio frequency signals with the frequency modulatedradio frequency signals transmitted by the originating transmitter topreserve signal integrity and stability.

Another object of the present invention is to provide an economicalarrangement for intermediate frequency repeating transmitter links.

A feature of the present invention is to obviate need for an additionalsubcarrier frequency which has been heretofore used for transmitting asynchronizing tone to lock the carrier frequency of the FM transmitter.

Another feature of the present invention is that the modulation levelsat all remote transmitters are synchronized.

Another feature of the present invention is the minimization of audiophase delays for enhancing stereo quality.

Another feature of the present invention is the elimination of an FMsubcarrier to transmit a reference signal for synchronizing thefrequency of the booster transmitter.

An object of the present invention is to provide a frequency modulatedratio frequency broadcast network having an originating frequencymodulated radio frequency transmitter and a re-transmitting frequencymodulated radio frequency transmitter with a booster that reduces phasenoise in the re-transmitted frequency modulated broadcast carrier toimprove the signal quality in the overlap carrier areas of theoriginating frequency modulated radio frequency transmitter and there-transmitting frequency modulated radio frequency transmitter.

A feature of the present invention is a cleaner spectrum in there-transmitted broadcast frequency modulated carrier by the directgeneration of the re-transmitted broadcast frequency modulated carrierfrom a voltage controlled oscillator in the FM exciter of there-transmitting frequency modulated ratio frequency transmitter.

Another feature of the present invention is the achievement of improvedaudio quality resulting from the elimination of the demodulation andreproduction process, which normally occurs in an STL-Exciter programtransmission system.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a transmitter system for originatingfrequency modulated radio frequency signals and transmitter systems forre-transmitting frequency modulated radio frequency signals.

FIG. 2 is a block diagram of a modification of the transmitter systemshown in FIG. 1.

FIG. 3 is a block diagram of a further modification of the transmittersystem shown in FIG. 1.

FIG. 4 is a block diagram of a still further modification of thetransmitter system shown in FIG. 1.

FIG. 5 is a schematic diagram of a synchronous FM exciter employed inthe transmitter system shown in FIGS. 1-4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Illustrated in FIG. 1 is a serial feed frequency modulated radiofrequency broadcast system or network 10 with boosters, which comprisesan originating broadcast station 15 for broadcasting an originatingfrequency modulated radio frequency signal (FM) and re-transmittingbroadcast stations 20 and 25 for re-transmitting, respectively, thefrequency modulated radio frequency signals (FM) transmitted by theoriginating broadcast station 15. The re-transmitting broadcast stations20 and 25 are on-site remotely located from the site of the originatingbroadcast station 15.

The originating FM broadcast station 15 comprises a suitable studio 30,a conventional studio transmitter link (STL) receiver 35 with anintermediate frequency output, a synchronous FM exciter 40, and aconventional originating FM transmitter 45. Included in the studio 30are an FM broadcast stereo generator 46, and conventional FM subcarriergenerators for subsidiary communication authorization (SCA) equipment 47and 48, all of which are connected to a conventional studio link (STL)transmitter 50 for transmitting a composite FM baseband signal from thestudio to the originating broadcast station 15.

The FM broadcast stereo generator 46 is modified to accept a highlystable time base reference signal. This signal is 19 KHz or multiple of19 KHz generated by a highly stable crystal oscillator, not shown, witha long term stability of 1 ppm per year. The highly stable crystaloscillator is included in the STL transmitter 50. In this manner, ahighly stable time base reference signal is provided for phase lockingthe originating broadcast frequency and the rebroadcasting frequenciesin the entire network 10 so that stability and accuracy of the broadcastfrequencies are achieved.

The output of the STL transmitter 50 is an FM signal which, in theexemplary embodiment, is a 950 megahertz (MHz) signal. The frequencydeviation (modulation level) of the STL transmitter is 50%±37.5 MHz ofthe FM broadcast carrier, which, in the exemplary embodiment, is 88MHz-108 MHz. The stereo generator 46 and the SCA equipment 47 and 48modulate the carrier frequency generated by the STL transmitter 50 withthe program to be broadcast to provide the FM signal of 950 MHz. The STLtransmitter 50 applies a reference signal to the stereo generator 46. Inthe exemplary embodiment, the reference signal is a 19 kilohertz (KHz)signal or a multiple of a 19 KHz frequency signal.

The STL receiver 35 receives through a suitable studio link 55, the FMsignal from the STL transmitter 50, and applies an FM signal to thesynchronous FM exciter 40 and an FM signal to a conventionalintermediate frequency (IF) STL transmitter or a one-way repeater 60. Inthe exemplary embodiment, the FM signal applied to the synchronous FMexciter 40 from the STL receiver 35 is a 2.5 MHz signal and the FMsignal applied to the IF repeater STL transmitter 60 is a 63 MHz signal,which is also the IF signal of the STL receiver 35. The studio link 55,in the exemplary embodiment, may be a conventional microwave link,telephone lines, or the like. The reference signal from the stereogenerator 46 to the STL transmitter 50 is received by the STL receiver35 with the 950 MHz signal and is present in both the 2.5 MHz signalapplied to the synchronous FM exciter 40 and the 63 MHz signal appliedto the IF STL transmitter 60.

The synchronous FM exciter 40, which will be described in detailhereinafter, applies to the originating FM transmitter 45, an FMbroadcast signal, the carrier frequency of which is generated by thesynchronous FM exciter 40. In the exemplary embodiment, the FMtransmitter 45 transmits an FM broadcast signal in the range of 88MHz-108 MHz.

A conventional link 65, such as a microwave link, VHF or UHF link,inter-city relay and the like is employed for transmitting an FM IFsignal to an STL receiver 70 of the re-transmitting FM broadcast boosterstation 20. In the exemplary embodiment, the FM IF signal transmitted bythe IF STL transmitter 60 is an FM 950 MHz signal. The IF STLtransmitter 60 increases the carrier frequency from 63 MHz to 950 MHz,in the exemplary embodiment.

The STL receiver 70 applies an FM IF signal to a synchronous FM exciter75 of the re-transmitting station 20 and an FM IF signal to an IFrepeater STL transmitter or one-way repeater 80 of the re-transmittingstation 20. The signal applied to the synchronous FM exciter 75, in theexemplary embodiment, is 2.5 MHz FM signal with a 19 KHz referencesignal in the composite baseband signals. The signal applied to the IFrepeater STL transmitter 80, in the preferred embodiment is 63 MHz FMsignal with a 19 KHz reference signal in the composite baseband signalswhich is also the IF signal of the STL receiver 70. The synchronous FMexciter 75, which will be described in detail hereinafter, is similar inconstruction and operation to the synchronous FM exciter 40 of theoriginating transmission station 15. In the exemplary embodiment, theoutput of the synchronous FM exciter 75 is an FM signal having afrequency in the range of 88 MHz-108 MHz.

Connected to the output of the synchronous FM exciter 75 is a suitablebooster 85 of the re-transmission station 20, which, in a conventionalmanner, amplifies the power of the FM signal applied to are-transmitting or repeater transmitter 90 of the re-transmittingstation 20. Thus, the output of the FM signal from the synchronous FMexciter 75 has the power level thereof amplified by the booster 85 tocompensate for losses that may occur from the transmission of signalsfrom the broadcast station 15 to the broadcast station 20 and increasethe signal strength of the originating transmitter without causingtransmission interference with other booster stations of the network 10.The output signal of the re-transmitter 90, in the preferred embodiment,is in the range of 88 MHz-108 MHz.

A conventional link 95, such as a microwave link, VHF or UHF link,inter-city relay and the like is employed for transmitting an FM IFsignal to an STL receiver 100 of the re-transmitting FM broadcastbooster station 25. In the exemplary embodiment, the FM IF signaltransmitted by the IF repeater STL transmitter 80 is an FM 950 MHzsignal. The IF repeater STL transmitter 80 increases the carrierfrequency from 63 MHz to 960 MHz in the exemplary embodiment.

The STL receiver 100 applies an FM signal to a synchronous FM exciter105 of the re-transmitting station 25 and an FM signal to an IF repeaterSTL transmitter or one-way repeater 110 of a succeeding station. Thesignal applied to the synchronous FM exciter 105, in the exemplaryembodiment, is a 2.5 MHz FM signal with a 19 KHz reference signal in thecomposite baseband frequencies. The signal applied to the IF repeaterSTL transmitter 110, in the preferred embodiment, is a 63 MHz FM signalwith a 19 KHz reference signal in the composite baseband signals whichis also the IF signal of the STL receiver 100. The synchronous FMexciter 105, which will be described in detail hereinafter, is similarin construction and operation to the synchronous FM exciter 75 of there-transmission station 20. In the exemplary embodiment, the output ofthe synchronous FM exciter 105 is an FM signal having a frequency in therange of 88 MHz-108 MHz.

Connected to the output of the synchronous FM exciter 105 is a suitablebooster 115 of the re-transmission station 20, which, in a conventionalmanner, amplifies the power of the FM signal applied to are-transmitting or repeater transmitter 120 of the re-transmittingstation 25. Thus, the output of the FM signal from the synchronous FMexciter 105 has the power level thereof amplified by the booster 115 tocompensate for losses that may occur from the transmission of signalsfrom the originating broadcast station 15 to the rebroadcast station 25and, perhaps, increase the signal strength of the originatingtransmitter 15 without causing transmission interference with otherbooster stations of the network 10. The output signal of there-transmitter 120, in the preferred embodiment, is in the range of 88MHz-108 MHz.

In the FM broadcast network 10, the studio STL transmitter 50 is incommunication with the originating broadcast station 15, but is isolatedfrom the FM rebroadcast stations 20 and 25 or and succeeding rebroadcaststation of the network 10. The FM signal is transmitted successivelyfrom one transmitter to the succeeding transmitter in a serial fashionstarting with the originating transmitter 45 and proceeding successivelyor in series in the order of proximity to the originating transmitter45.

Illustrated in FIG. 2 is a parallel feed frequency modulated radiofrequency broadcast system or network 125 with boosters, which is amodification of the FM radio frequency broadcast system or network 10shown in FIG. 1. Elements of the FM radio frequency system 125 similarin construction and operation to the elements of the FM radio frequencysystem or network 10 are designated with the same reference numeral butwith a prime suffix.

The FM radio frequency broadcast system 125 differs from the FM radiofrequency broadcast system 10 in that the IF repeater STL transmitter 60has been omitted. As a consequence thereof, the STL transmitter 50' hasthe output thereof transmit the baseband composite signals directly tothe STL receiver 70' of the re-transmitting broadcast station 20'. Thus,the originating transmitter 45' is boosted by the re-transmittingstations 20', 25' and any succeeding re-transmission station. In thisarrangement, the STL transmitter 50' of the studio 30' of theoriginating broadcast station 10' applies an FM signal to the STLreceiver 35' of the originating broadcast station 10' and to thesucceeding STL receiver 70' of the re-transmitting broadcast station20'. Thus, the STL transmitter 50' applies an FM signal to theoriginating transmitter station 15' and the succeeding re-transmittingand repeater station 20'. The re-transmitting station 25' operates inthe manner heretofore described in connection with the re-transmittingstation 25 in FIG. 1 and its reception in relation to re-broadcaststation 20' is similar to that heretofore described in relation tore-broadcast station 25 and re-broadcast station 20 of FIG. 1.

Illustrated in FIG. 3 is a parallel and serial feed FM radio frequencybroadcast system or network 130 with boosters, which is a furthermodification of the FM radio frequency broadcast system 10 shown inFIG. 1. Elements of the FM radio frequency system 130 or network similarin construction and operation to the elements of the radio frequencysystem 10 are designated with the same reference numeral but with adouble prime suffix.

The FM radio frequency broadcast system 130 differs from the FM radiobroadcast system 10 in that the STL transmitter 50" of the studio 30"for the originating broadcast station 15" has the output thereofadditionally transmit the baseband composite signals to the STL receiver100" of the re-transmitting station 25" through a suitable link 135.Thus, the input of the IF repeating STL receiver 100" of there-transmitting station 25" does not receive the output of the IFrepeater STL receiver 70" of the re-transmitting station 20' butreceives the output of the STL transmitter 50" of the studio 30" of theoriginating broadcast station 15" through the link 135. The link 135 maybe a microwave link, VHF or UHF link, inter-city relays or the like.

The FM broadcast system 130 shown in FIG. 3 is employed when the terrainisolates the FM re-transmitting system 25" from the FM re-transmittingsystem 20". In this arrangement, the STL transmitter 50" in the studio30" sends an FM signal to the originating broadcast station 15" and tothe nearest unobstructed re-transmitting broadcast station 20" and theoriginating broadcast station 15". The FM signal is then relayed to theobstructed re-transmitting broadcast station 25" from the STLtransmitter 50" at the originating broadcast station 15".

Illustrated in FIG. 4 is an FM radio frequency broadcast system ornetwork 140 with boosters, which is a still further modification of theFM radio frequency broadcast system or network 10. Elements of the FMradio frequency system or network 140 similar in construction andoperation to the elements of the radio frequency system or network 10are designated with the same reference numeral but with a triple primesuffix.

The FM radio frequency broadcast system or network 140 differs from theFM radio frequency broadcast system or network 10 in that the referencetime base signal from the STL receiver 35"' is applied over a separatepath to an FM modulating exciter 40a. An IF repeater STLreceivertransmitter 60a is located at a relay point between theoriginating transmitter station 15"' and the booster station 20"'. Inthe exemplary embodiment, the FM signal received by the IF repeater STLreceiver-transmitter 60a from the originating transmitters 45"' is inthe range of 88 MHz-108 MHz. Similarly, the IF signal from the STLreceiver 70"' is applied to the synchronous FM exciter 75"'. The FMexciter 40a is a modification of a conventional FM broadcast exciter andis arranged to accept an external time base reference signal for phaselocking the broadcast frequency carrier in the range of 88-108 MHz. Theexternal time base reference signal is in the frequency range of 2.5 KHzto 19 KHz and is generated from a conventional stable crystaloscillator, not shown, having a long term stability of a ppm per year.The time base reference signal for locking the FM broadcast exciter 40ais generated in the STL receiver 35"'.

Illustrated in FIG. 5 is the synchronous FM exciter 75 shown in blockdiagram in FIG. 1. The synchronous FM exciters 40 and 105 are similar inconstruction and operation to the synchronous FM exciter 75 and, hence,only the synchronous FM exciter 75 will be described in detail. Itfollows that the synchronous FM exciters 40', 75', 105', 40", 75", 105",and 75"' are similar in construction and operation to the synchronous FMexciter 75.

The synchronous FM exciter 75 has applied thereto from the STL receiver70 an FM IF signal, which, in the exemplary embodiment, is a 2.5 MHz FMIF signal modulated by the composite signals for the FM broadcastingincluding a reference signal of 19 KHz, in the exemplary embodiment. Thepeak frequency deviation of the 2.5 MHz signal is ±37.5 KHz. The 2.5 MHzFM IF signal is applied to a conventional FM discriminator 141, whichdemodulates the 2.5 MHz signal into the reference signal of 19 KHz. The19 KHZ reference signal is applied to one input of a conventional phasedetector 142 of a phase locked loop circuit 143 through a suitablebandpass filter 141a. The FM discriminator 141 and the bandpass filter141a is part of the phase locked loop circuit 143.

A suitable reference signal is generated by a conventional voltagecontrolled crystal oscillator (VCXO) 145 of the phase locked loopcircuit 143. The output of the VCXO 145 is applied to a conventionalfrequency divider circuit 146 of the phase locked loop circuit 143. Theoutput frequency of the VCXO 145, in the exemplary embodiment, is 15.2MHz. The output frequency of the frequency divider network 146 which is,in the exemplary embodiment, a 19 KHz reference signal, is applied tothe other input of the phase detector 142.

The phase detector 142 produces in the output thereof an error signalwhose amplitude is proportional to the difference in phase between theinput signals applied to the phase detector 142. The phase error signalis applied to a conventional phase error amplifier 150 of the phaselocked loop circuit 143. The phase error signal produced in the outputof phase error amplifier 150 is applied to the VCXO 145 to compensatefor the phase error or difference applied to the inputs of the phasedetector 142.

When the phase difference of the reference signals applied to the inputsof the phase detector 142 approaches or approximates zero, the VCXO 145is phase locked to the 19 KHz reference signal contained in thecomposite signal in the output of the FM discriminator 141. The stablereference signal in the output of the VCXO 145 is now phase locked withthe 19 KHz reference signal of the composite signal in the output of theFM discriminator 141 and the IF frequency modulated signal sent by theoriginating broadcast station 15.

Another output of the frequency divider circuit 146 is applied toconventional frequency divider networks 154 and 154a of the phase lockedloop circuits 155a and 155b, respectively. In the exemplary embodiment,the output frequency of the VCXO 145 applied to the divider circuit 146is 15.2 MHz. The reference signal in the output of the frequency dividercircuit 146, in the exemplary embodiment, is 80 KHz. The referencesignal in the output of the respective frequency divider networks 154and 154a, in the exemplary embodiment, is 10 KHz. The 10 KHz referencesignals of the divider networks 154 and 154a are applied respectively toone input of a conventional phase detector 165 of the phase locked loopcircuit 155a and to one input of a conventional phase detector 166 ofthe phase locked loop circuit 155b.

The FM IF signal from the STL receiver 70 is applied to a conventionalfrequency doubler circuit 167. Thus, the output of the frequency doublercircuit 167, in the exemplary embodiment, is 5 MHz. The frequencydoubler circuit 167 multiplies the peak frequency deviation of the STLmodulation to ±75 KHz. The output of the frequency doubler circuit 167is applied to one input of a conventional phase detector circuit 168 ofthe phase locked loop circuit 155c through a suitable linear phasebandpass filter 167a and a suitable amplifier 167b.

The phase error signal of the output of the phase detector 168 isapplied to a conventional phase error amplifier 169 of the phase lockedloop circuit 155c. The phase detector 168 produce's in its output anerror signal whose amplitude is proportional to the difference in phasebetween the input signals applied to the phase detector 168. The phaseerror signal is applied to the phase error amplifier 169. The phaseerror signal produced in the output of the phase error amplifier 169 isapplied to a conventional voltage controlled oscillator (VCO) 170 of thephase locked loop circuit 155c to compensate for the phase error ordifference applied to the inputs of the phase detector circuit 168.

When the phase differences of the reference and comparison signalsapplied to the inputs of the phase detector 168 approaches orapproximates zero, the VCO 170 is phase locked to the 5 MHz referencesignal in the output of the frequency doubler circuit 167. The VCO 170generates the FM broadcast carrier, which, in the exemplary embodiment,is between 88 MHz and 108 MHz. Thus, the phase locked loop circuit 155cis the phase locked loop circuit that phase locks the VCO 170 to thereference signal derived from the FM IF signal emanating from the STLreceiver 70, which, in the exemplary embodiment, is 2.5 MHz.

One output of the VCO 170 is applied to a conventional frequency dividercircuit 162 of the phase locked loop circuit 155a. The frequency dividercircuit 162 is a programmable divider circuit and produces in its outputa 10 KHz signal, in the exemplary embodiment, which is applied to oneinput of a conventional phase detector circuit 165 of the phase lockedloop circuit 155a. Applied to the other input of the phase detectorcircuit 165 is the output frequency of the frequency divider circuit154, which in the exemplary embodiment is 10 KHz. The output signal ofthe frequency divider circuit 154 applied to the phase detector circuit165 is derived from the stable output reference signal derived from theVCXO 145 of the phase locked loop circuit 143.

The phase detector circuit 165 produces in its output an error signalwhose amplitude is proportional to the difference in phase between theinput signals applied to the phase detector 165. The phase error signalis applied to a conventional phase error amplifier 195 of the phaselocked loop circuit 155a. The phase error signal produced in the outputof the phase error amplifier 195 is applied to a conventional voltagecontrolled crystal oscillator (VXCO) 196 of the locked loop circuit155a.

When the phase differences of the reference and comparison signalsapplied to the inputs of the phase detector circuit 165 approaches orapproximates zero, the stable reference signal in the output of the VCXO196 is now phase locked with the stable reference output signal of theVCXO 145 through the frequency divider circuits 146 and 154. The outputof the VCXO 196 is applied to one input of a conventional phase detectorcircuit 198 of the phase locked loop circuit 155a.

In the exemplary embodiment, the output signal produced by a voltagecontrolled oscillator (VCO) 156 of the phase locked loop circuit 155a isin the range of 144 MHZ-168 MHz. One output signal of the VCO 156 isapplied to a conventional frequency divider circuit 197 of the phaselocked loop circuit 155a. In the exemplary embodiment, the dividercircuit 197 is a ÷16 divider. The output of the frequency dividercircuit 197 is applied to one input of a conventional phase detectorcircuit 198 of the phase locked loop circuit 155a. The output of theVCXO 196 is applied to the other input of the phase detector circuit198. The output of the phase detector circuit 198 is applied to aconventional phase error amplifier 199 of the phase locked loop circuit155a.

The phase detector circuit 198 produces in the output thereof an errorsignal whose amplitude is proportional to the differences in phasebetween the input signals applied to the phase detector 198. The phaseerror signal in the output of the phase detector circuit 198 is appliedto the phase error amplifier 199. The phase error signal produced in theoutput of the phase error amplifier 199 is applied to the VCO 156 tocompensate for the phase error or phase difference applied to the phasedetector circuit 198.

When the phase difference of the input signals applied to the inputs ofthe phase detector 198 approaches or approximates zero, the VCO 156 isphase locked to the stable reference signal of the VCXO 196. The outputfrequency of the VCXO 196 is phase locked to and corrected by the stablereference signal of the VCXO 145. The output frequency signal of the VCO156 serves to correct the frequency error of the FM IF frequency signalapplied to the VCO 170 in a manner hereinafter described. Theprogrammable frequency divider circuit 162 provides a rapid synthesizedreference signal in the output of the VCO 156 for correcting phasedifferences between the output frequency of the VCO 170 and the phaselocked frequency reference signal produced in the output of the doublercircuit 167.

The output of the VCO 156 is applied to one input of a conventionalmixer circuit 160 of the phase locked loop circuit 155c. The output ofthe VCO 170 is applied to the other input of the mixer circuit 160. Theoutput frequency of the mixer circuit 160, in the exemplary embodiment,is 50 MHz-65 MHz, which is applied to one input of a conventional mixercircuit 200 of the phase locked loop circuit 155 through a conventionalbandpass filter 160a. The phase locked loop circuit 155a is a fastresponding loop (low noise) to provide a programmable synthesizedreference signal so that the frequency of the VCO 170 can be converteddown to a 5 MHz signal for the phase locked loop circuit 155c. The phaselocked loop circuit 155c is phase locked to the output frequency of thedoubler circuit 167 through the bandpass filter 167a and the amplifier167b.

The output of the frequency divider network 154a is applied to one inputof a phase detector circuit 166 of a phase locked loop circuit 155b. Theoutput frequency of the frequency divider network 154a, in the exemplaryembodiment, is 10 KHz. One output of a conventional voltage controlledoscillator (VCO) 201 of the phase locked loop circuit 155b is applied toa conventional frequency divider circuit 202 of the phase locked loopcircuit 155b. The frequency divider circuit 202 is a programmabledivider circuit. In the exemplary embodiment, the signal generated bythe VCO 201 is in the frequency range of 55 MHz-70 MHz. The outputsignal of the frequency divider circuit 202 is applied to the otherinput of the phase detector circuit 166. The programmable frequencydivider circuit 202 provides a rapid synthesized reference signal in theoutput of the VCO 201 for correcting phase differences between theoutput of the VCO 201 and the phase locked frequency reference signalproduced in the output of the doubler circuit 167.

The output of the phase detector circuit 166 is applied to aconventional phase error amplifier 203 of the phase locked loop circuit155b. The phase detector circuit 166 produces in the output thereof anerror signal whose amplitude is proportional to differences in phasebetween the input signals applied to the input circuits of the phasedetector circuit 166. The phase error signal produced in the output ofthe phase error amplifier 203 is applied to the VCO 201 to compensatefor the phase error or phase difference in the output of the phasedetector 166. When the phase difference of the signals applied to theinput circuits of the phase detector 166 approaches or approximateszero, the VCO 201 is phase locked to the VCXO 145 through the frequencydivider circuit 16 and the divider network 154a. The output of the mixercircuit 200 is applied to the other input circuit of the phase detector168 through a conventional amplifier 200a of the phase locked loopcircuit 155c. The VCO 170 is phase locked in a manner previouslydescribed to the output frequency of the doubler circuit 167 through thebandpass filter 167a and the amplifier 167b. The VCO 201 is phase lockedto the stable output frequency of the VCXO 145, which, in turn, is phaselocked to a reference signal derived from the FM IF signal applied tothe descriminator 141.

The other output of the VCO 201 is applied to a conventional mixercircuit 200 of the phase locked loop circuit 155c. In the exemplaryembodiment, the output of the mixer circuit 200 is 5 MHz, which isderived from the difference between the 50 MHZ-65 MHz output frequencyof the VCO 156 and the 55 MHz-70 MHz output frequency of the VCO 201.The output of the VCO 170 produces the FM broadcast signal fortransmission, which, in the exemplary embodiments, is in the range of 88MHz-108 MHz.

The phase locked loop circuits 143 and 155 serve to maintain modulationand frequency integrity for the re-transmission station 20. The stations15, 20 and 25 are synchronized to eliminate interference. Heterodyne andnoise to the unsynchronized transmitters, phase shifts on the baseband,group delays and inconsistent modulation levels have been obviated toenhance broadcast program quality. A cleaner spectrum in the broadcastFM carrier is achieved by direct generation of the carrier from the VCO170 instead of filtering out the FM broadcast carrier after heterodyneof two signals. The VCO 170 is operating at the desired output frequencyat a high output level. Hence, additional amplification is not requiredfor driving an additional power amplifier. The FM broadcast outputsignal from the power amplifier 172 is applied to the booster 85. The FMbroadcast output signal of the booster 85 is applied to there-transmitter 90 for broadcasting the FM signal that has originatedfrom the broadcast studio 30.

A conventional automatic level control (ALC) circuit 175 is coupled tothe output of the power amplifier 172 and applies a compensating controlvoltage to the power amplifier 172 to automatically maintain poweroutput level applied to the booster 85. Included in the ALC circuit 175is an adjustable variable resistor 176 for setting a reference voltageapplied to the level control circuiting of power amplifier 172 in awell-known manner.

While reference herein may be made to standard FM broadcast frequencysignals, it appears that the invention disclosed herein is alsoapplicable to any FM radio frequency signals, including VHF, UHF andmicrowave signals, FM inter-city relay link signals, and any FMbroadcast signals. It is also apparent that the synchronous FM excitersfor the originating broadcast stations 15, 15', and 15" need not employall the operational features of the synchronous FM exciter 75, whichwere described in detail.

What is claimed is:
 1. A synchronous frequency modulated booster forre-transmitting a frequency modulated broadcast signal comprising:A.transmitting means for re-transmitting a frequency modulated broadcastsignal; B. receiving means for receiving a frequency modulatedintermediate frequency signal .Iadd.modulated by a frequency referencesignal.Iaddend.; and C. a synchronous frequency modulated exciterreceiving said frequency modulated intermediate frequency signal fromsaid receiving means and applying to said transmitting means a frequencymodulated broadcast signal for re-transmission by said transmittermeans; D. said synchronous frequency modulated exciter comprising:(a)means responsive to said receiving means for producing a frequencyreference signal representative of the frequency modulated intermediatefrequency signal, (b) a first phase locked loop circuit responsive tosaid receiving means for producing a first stable phase locked frequencysignal representative of the frequency modulated intermediate frequencysignal, (c) a second phase locked loop circuit including a first voltagecontrolled oscillator, said first voltage controlled oscillatorgenerating the frequency modulated broadcast signal, said first voltagecontrolled oscillator being phase locked to said frequency referencesignal produced by said means, and (d) correcting phase locked loopcircuit means responsive to the output of said first voltage controlledoscillator and responsive to said stable phase locked frequency producedby said first phase locked loop circuit and connected to said secondphase locked loop circuit for correcting phase differences between theoutput of said first voltage controlled oscillator and said frequencyreference signal produced by said means.
 2. A synchronous frequencymodulated booster for re-transmitting a frequency modulated broadcastsignal comprising:A. transmitting means for re-transmitting a frequencymodulated broadcast signal; B. receiving means for receiving a frequencymodulated intermediate frequency signal .Iadd.modulated by a frequencyreference signal.Iaddend.; and C. a synchronous frequency modulatedexciter receiving said frequency modulated intermediate frequency signalfrom said receiving means and applying to said transmitting means afrequency modulated broadcast signal for re-transmission by saidtransmitter means; D. said synchronous frequency modulated excitercomprising:(a) means responsive to said receiving means for producing afrequency reference signal representative of the frequency modulatedintermediate frequency signal, (b) a first phase locked loop circuitresponsive to said receiving means for producing a first stable phaselocked frequency signal representative of the frequency modulatedintermediate frequency signal, (c) a second phase locked loop circuitincluding a first voltage controlled oscillator, said first voltagecontrolled oscillator generating the frequency modulated broadcastsignal, said first voltage controlled oscillator being phase locked tosaid frequency reference signal produced by said means, and (d)correcting phase locked loop circuit means responsive to the output ofsaid first voltage controlled oscillator and responsive to said stablephase locked frequency produced by said first phase locked loop circuitand connected to said second phase locked loop circuit for correctingphase differences between the output of said first voltage controlledoscillator and said frequency reference signal produced by said means,(e) said second phase locked loop circuit including a first mixercircuit, one input of said first mixer circuit being connected to theoutput of said first voltage controlled oscillator, said correctingphase locked loop circuit means including a second voltage controlledoscillator, said second voltage controlled oscillator being connected toanother input of said first mixer circuit producing a signal forcorrecting phase differences between the output of said first voltagecontrolled oscillator and said frequency reference signal produced bysaid means.
 3. A synchronous frequency modulated booster as claimed inclaim 2 wherein said correcting phase locked loop circuit means includesa third phase locked loop circuit, said third phase locked loop circuitincludes said second voltage controlled oscillator and a first voltagecontrolled crystal oscillator, said first voltage controlled crystaloscillator producing a second stable frequency reference signal forphase locking said second voltage controlled oscillator, said firstphase locked loop circuit includes a second voltage controlled crystaloscillator for producing said first stable phase locked frequencysignal, said first voltage controlled crystal oscillator being phaselocked to the first stable frequency reference signal produced by saidsecond voltage controlled crystal oscillator.
 4. A synchronous frequencymodulated booster as claimed in claim 3 wherein said third phase lockedloop circuit includes a programmable frequency divider circuit toprovide a rapid synthesized reference signal in the output of saidsecond voltage controlled oscillator for correcting phase differencesbetween the output of said second voltage controlled oscillator and saidphase locked frequency reference signal produced by said first voltagecontrolled crystal oscillator.
 5. A synchronous frequency modulatedbooster as claimed in claim 4 wherein said second phase locked loopcircuit includes a second mixer circuit and wherein said correctingphase locked loop circuit means includes a fourth phase locked to loopcircuit, the output of said first mixer circuit being connected to oneinput of said second mixer, said fourth phase locked loop circuitincludes a third voltage controlled oscillator, said third voltagecontrolled oscillator being connected to another input of said secondmixer circuit, said second mixer circuit in response to the outputfrequency signal of said first mixer circuit and in response to theoutput frequency of said third voltage controlled oscillator producing afrequency signal to correct phase differences between the output of saidfirst voltage controlled oscillator and said frequency reference signalproduced by said means.
 6. A synchronous frequency modulated booster asclaimed in claim 5 wherein said third voltage controlled oscillator isphase locked in said first stable phase locked frequency signal producedby said second voltage controlled crystal oscillator.
 7. A synchronousfrequency modulated booster as claimed in claim 5 wherein said fourthphase locked circuit includes a programmable frequency divider circuitto provide a rapid synthesized reference signal in the output of saidthird voltage controlled oscillator for correcting phase differencesbetween the output of said third voltage controlled oscillator and saidphase locked frequency reference signal produced by said first phaselocked loop circuit.
 8. A synchronous frequency modulated booster forre-transmitting a frequency modulated broadcast signal comprising:A.transmitting means for re-transmitting a frequency modulated broadcastsignal; B. receiving means for receiving a frequency modulatedintermediate frequency signal .Iadd.modulated by a frequency referencesignal.Iaddend.; and C. a synchronous frequency modulated exciterreceiving said frequency modulated intermediate frequency signal fromsaid receiving means and applying to said transmitting means a frequencymodulated broadcast signal for re-transmission by said transmittermeans; D. said synchronous frequency modulated exciter comprising:(a)first means responsive to said receiving means for producing a frequencyreference signal representative of the frequency modulated intermediatefrequency signal, (b) a phase locked loop circuit including a voltagecontrolled oscillator, said voltage controlled oscillator generating thefrequency modulated broadcast signal, said voltage controlled oscillatorbeing phase locked to said frequency reference signal produced by saidfirst means, and (c) second means including a phase error correctingcircuit connected to the output of said voltage controlled oscillatorand said phase locked loop circuit and responsive to said receivingmeans for correcting phase differences between the output of saidvoltage controlled oscillator and said frequency reference signalproduced by said first means.
 9. A synchronous frequency modulatedbooster as claimed in claim 8 wherein said first means includes adoubler circuit responsive to said receiving means for producing saidreference signal representative of the frequency modulated intermediatefrequency signal.
 10. A synchronous frequency modulated booster asclaimed in claim 8 wherein said phase error correcting circuit producesa first stable phase locking frequency signal for correcting phasedifferences between the output of the voltage controlled oscillator andsaid frequency reference signal produced by said first means, and saidsecond means produces a second stable phase locking frequency signal forphase locking said first stable phase locking frequency signal.
 11. Asynchronous frequency modulated booster as claimed in claim 10 whereinsaid second means includes a phase locked loop circuit for producingsaid second stable phase locking frequency signal, said last-mentionedphase locked loop circuit being responsive to said receiving means forproducing a phase locked frequency reference signal representative ofthe frequency modulated intermediate frequency signal.
 12. A synchronousfrequency modulated booster as claimed in claim 8 wherein said phaseerror correcting circuit includes a programmable frequency dividercircuit to provide a rapid synthesized reference signal for correctingphase differences between the output of the voltage controlledoscillator and the reference signal produced by said first means.